Purpose of PTFE inner sleeves?

I see some hot ends have inner ptfe sleeves/tubes, others don't. for instance makerbot uses it on their mk5,6

I'm just curious what the purpose is. wouldn't a sleeve block some of the heat from reaching the filliment and hurt it from melting?
is it an attempt to stop the heat from traveling up the tube? becuase the metal outer tube would tranfer heat upwards anyways no?
it just seems like one more part that can wear down, move out of place. but I'm trying to design a print head to learn a bit more and I'd like to know if these are a good way to go.

thanks everyone~!

-Adam

At the transition zone, where the plastic has passed its glass transition, but is not molten it is like rubber. That causes it to expand and grip the sides of the barrel. If the coefficient of friction is above a critical limit it will jam. The transition zone either has to be very slippery, hence PTFE, or have an increasing taper, like the draft on a mould.

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[www.hydraraptor.blogspot.com]

thanks nophead.
I don't understand the draft though. if it gets narrower wouldn't thatforce it to jam up more not less? how does that overcome it?

thanks

It has to get wider lower down, not narrower, see [hydraraptor.blogspot.co.uk] and [hydraraptor.blogspot.co.uk].

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[www.hydraraptor.blogspot.com]

ok that makes much more sense. thanks nophead.

nophead, sorry another question.

which did you find works better? the ptfe for slip or simply reaming out the metal hole? it seems like reaming it might not work as well, but would hold up longer. I've never had a ptfe inner sleeve wear out, but I imagine it could over time.

at the end of the day it just seems like one less part which is nice, so if it works I'd love to go that route..

An all metal extruder lasts forever but it needs a heatsink and a fan. One with PTFE can be smaller and lighter but the PTFE needs to be totally constrained inside PEEK. Thin walled PTFE pipe soon fails in my experience. It has to be something like 8mm OD as in the J-Head.

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[www.hydraraptor.blogspot.com]